A common technique for forming a pattern across a substrate is photolithography. Such comprises patterning actinic radiation across a photosensitive material (for instance, photoresist), and subsequently developing the photosensitive material to form a patterned mask. In subsequent processing, the patterned mask may be utilized during fabrication of components across the underlying substrate.
Photolithography may be utilized during fabrication of integrated circuitry by using a photomask or reticle to pattern the actinic radiation. Photolithography may also be used during fabrication of photomasks and reticles by using an electron beam, or other controlled form of actinic radiation. In applications in which photolithography is utilized during fabrication of integrated circuitry, the underlying substrate may be a semiconductor wafer (for instance, a monocrystalline silicon wafer); and in applications in which photolithography is utilized during fabrication of photomasks and reticles, the underlying substrate may be quartz or any other materials suitable for utilization in photomasks and reticles.
The term “photomask” is traditionally understood to refer to masks which define a pattern for an entirety of a wafer, and the term “reticle” is traditionally understood to refer to a patterning tool which defines a pattern for only a portion of a wafer. However, the terms “photomask” (or more generally “mask”) and “reticle” are frequently used interchangeably in modern parlance, so that either term can refer to a radiation-patterning tool that encompasses either a portion or an entirety of a wafer. For purposes of interpreting this disclosure and the claims that follow, the terms “reticle” and “photomask” are utilized interchangeably to refer to radiation-patterning tools that define patterns across some or all of a wafer.
Difficulties may be encountered in attempting to uniformly disperse photosensitive material (for instance, photoresist) across an underlying substrate. For instance, photoresist may be applied to the underlying substrate by flowing liquid photoresist across a top surface of the substrate while the substrate is spinning. The liquid material spreads radially outward from a center of the substrate toward an edge of the substrate to coat a top of the substrate. Ideally, all excess photoresist would be ejected from the edge of the substrate. In practice, however, some excess photoresist tends to collect along the edge of the substrate and to form a bead along such edge (with the bead frequently be referred to as an “edge bead”).
Various methods have been developed for removing edge beads, but such methods frequently produce unsatisfactory results. For instance, one method for removing edge beads is to utilize organic solvent to dissolve photoresist along the edge of the substrate, and to thereby dissolve the edge bead. Unfortunately, the photoresist may come out of solution as the solvent evaporates. Thus, the net effect may be that the edge bead is transferred from one location to another across the upper surface of the substrate, rather than being removed from such upper surface during utilization of solvent. It is desired to develop new methods for removing edge beads.